Blanketing metals and alloys at elevated temperatures with gases having reduced global warming potential

ABSTRACT

An improved method of processing a nonferrous metal and alloys of said metal using a blanketing gas having a global warming potential is provided. The improvement involves reducing the global warming potential of the blanketing gas by blanketing the nonferrous metal and alloys with a gaseous mixture including at least one compound selected from the group consisting of COF 2 , CF 3 COF, (CF 3 ) 2 CO, F 3 COF, F 2 C(OF) 2 , SO 2 F 2 , NF 3 , SO 2 ClF, SOF 2 , SOF 4 , NOF, F 2  and SF 4 .

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of application Ser.No. 09/499,593, entitled “Blanketing Molten Nonferrous Metals and AlloysWith Gases Having Reduced Global Warming Potential,” filed Feb. 7, 2000.

BACKGROUND OF THE INVENTION

[0002] The present invention pertains to the blanketing of metals andalloys with gaseous mixtures, and in particular to a method ofblanketing metals and alloys at elevated temperatures using gases havingreduced global warming potentials relative to the prior art.

[0003] Open top vessels such as crucible and induction furnaces used tomelt nonferrous metals are operated so that the surface of metal duringmelting and the surface of the molten bath are exposed to ambientatmosphere. Air in the atmosphere tends to oxidize the melt, thereby:causing loss of metal, loss of alloying additions and formation of slagthat causes difficulty in metal processing; shortening refractory life;and promoting nonmetallic inclusions in final castings, pickup ofunwanted gases in the metals, porosity, and poor metal recovery. Onesolution is to enclose the melt furnace in a vacuum or atmospherechamber for melting and/or processing of the metals. However, completelyenclosed systems are very expensive and limit physical and visual accessto the metals being melted.

[0004] As alternatives, liquid fluxing salts, synthetic slag, charcoalcovers, and similar methods and compounds have been used in thehigh-volume, cost-sensitive field of metal reprocessing for minimizingmetal oxidation, gas pickup, and loss of alloying additions. Forexample, the prior art teaches that rapid oxidation or fire can beavoided by the use of fluxes that melt or react to form a protectivelayer on the surface of the molten metal. However, this protective layerof thick slag traps good metal, resulting in a loss of up to 2% of themelt. It also can break up and be incorporated into the melt, creatingdamaging inclusions. In addition, metal in the slag is leachable andcreates a hazardous waste product.

[0005] These prior art techniques also necessitate additional handlingand processing, and cause disposal problems. These techniques oftenreduce furnace life or ladle refractory life, increase frequency ofshutdowns for relining or patching of refractories, and producenon-metallic inclusions that have to be separated from the metal bathprior to pouring of the metal into a cast shape.

[0006] In searching for solutions to the above-described problems,metallurgical industries turned to inert gas atmosphere blanketing. Onetype of gas blanketing system is based on gravitational dispersion ofcryogenically-liquified inert gas over the surface of a hot metal to beblanketed. For example, such cryogenic blanketing systems are disclosedand claimed in U.S. Pat. No. 4,990,183.

[0007] U.S. Pat. No. 5,518,221 discloses a method and apparatus forinerting the interior space of a vessel containing hot liquids or solidsin induction furnaces, crucible furnaces or ladles during charging,melting, alloying, treating, superheating, and pouring or tapping ofmetals and metal alloys. The method and apparatus employ a swirl ofinert gas to blanket or cover the surface of the metal from the time ofcharging of the furnace until the furnace is poured or tapped orinerting of the molten metal contained in a furnace or ladle or othervessel. The gas swirl is confined by a unique apparatus mounted on topof the furnace or vessel containing the material to be protected. Anyinert gas that is heavier than air can be used to practice theinvention. In addition to argon and nitrogen, depending upon thematerial being blanketed, gases such as carbon dioxide and hydrocarbonsmay be used.

[0008] While some cryogenic blanketing systems are quite effective, useof such systems is limited to metallurgical facilities and vessels thatcan be supplied by well-insulated cryogenic pipelines or equipped withcryogenic storage tanks in close proximity to the point of use of theliquid cryogen. This is not always practical, and some cryogenicblanketing systems have been plagued by poor efficiency due to prematureboil-off of the cryogenic liquid and oversimplified design of dispersingnozzles that wasted the boiled-off gas.

[0009] Moreover, cryogenic dispensers often fail to uniformly dispersethe cryogenic liquid over the blanketed surface, leading to a transientaccumulation or entrapment of the liquid in pockets under the slag ordross, which may result in explosions in a subsequent rapid boil-off.

[0010] Other approaches have been taken for different molten metals andalloys in further attempts to solve the above-described problems. Forexample, U.S. Pat. No. 4,770,697 discloses a process for protecting analuminum-lithium alloy during melting, casting and fabrication ofwrought shapes by enveloping the exposed surfaces with an atmospherecontaining an effective amount of a halogen compound (e.g.,dichlorodifluoromethane) having at least one fluorine atom and one otherhalogen atom; the other halogen atom is selected from the groupconsisting of chlorine, bromine, and iodine, and the ratio of fluorineto the other halogen atom in the halogen compound is less than or equalto one. A passivating and self-healing viscous liquid layer is formedwhich protects the alloy from lithium loss due to vaporization,oxidation of the alloy, and hydrogen pick-up by the alloy.

[0011] Another approach for some molten metals, such as magnesium, is touse inhibitors in the air. The early practice was to burn coke or sulfurto produce a gaseous agent, CO₂ or SO₂. An atmosphere of CO₂ was foundto be superior to the commonly used commercial atmospheres of N₂, Ar, orHe because of the absence of vaporization of the magnesium, the absenceof excessive reaction products, and the reduced necessity for theenclosure above the molten metal to be extremely air tight.

[0012] However, the use of these inhibitors has several drawbacks. Forexample, both CO₂ and SO₂ pose environmental and health problems, suchas breathing discomfort for personnel, residual sludge disposal, and acorrosive atmosphere detrimental to both plant and equipment.Furthermore, SO₂ is toxic, corrosive, and can cause explosions.

[0013] While BF₃ has been mentioned as being a very effective inhibitor,it is not suitable for commercial processes because it is extremelytoxic and corrosive. Sulfur hexafluoride (SF₆) also has been mentionedas one of many fluorine-containing compounds that can be used in air asan oxidation inhibitor for molten metals, such as magnesium. A summaryof industry practices for using SF₆ as a protective atmosphere, ideasfor reducing consumption and emissions, and comments on safety issuesrelated to reactivity and health are provided in “Recommended Practicesfor the Conservation of Sulfur Hexafluoride in Magnesium MeltingOperations,” published by the International Magnesium Association (1998)as a “Technical Committee Report” (hereinafter “IMA Technical CommitteeReport”).

[0014] The use of pure SF₆ was generally discarded because of its severecorrosive attack on ferrous equipment. In addition, the use of pure SF₆for protecting molten metals such as magnesium has been reported to havecaused explosions. Although sulfur hexafluoride (SF₆) is consideredphysiologically inert, it is a simple asphyxiant which acts bydisplacing oxygen from the breathing atmosphere.

[0015] Later, it was found that at low concentrations of SF₆ in air(<1%), a protective thin film comprising MgO and MgF₂ is formed on themagnesium melt surface. Advantageously, even at high temperatures inair, SF₆ showed negligible or no reactions.

[0016] However, the use of SF₆ and air has some drawbacks. The primarydrawback is the release to the atmosphere of material having a highglobal warming potential (GWP).

[0017] It also was found that CO₂ could be used together with SF₆ and/orair. A gas atmosphere of air, SF₆, and CO₂ has several advantages.First, this atmosphere is non-toxic and non-corrosive. Second, iteliminates the need to use salt fluxes and the need to dispose of theresulting sludge. Third, using such an atmosphere results in lower metalloss, elimination of corrosion effects, and clean castings. Fourth, acasting process using such an atmosphere provides a clean operation andimproved working conditions. Fifth, the addition of CO₂ to theblanketing atmosphere reduces the concentration of SF₆ at which aneffective inerting film is formed on the metal. In sum, the addition ofCO₂ to an air/SF₆ atmosphere provides much improved protection comparedto the protection obtained with an air/SF₆ atmosphere.

[0018] However, using an atmosphere of SF₆ and CO₂ also hasdisadvantages. Both SF₆ and CO₂ are greenhouse gases, i.e., each has aglobal warming potential over 100 years (GWP₁₀₀). Thus, there is a needto reduce the amounts of SF₆ and CO₂ released into the atmosphere. SF₆has a 100-year global warming potential (GWP₁₀₀) of 23,900 relative toCO₂. International concern over global warming has focused attention onthe long atmospheric life of SF₆ (about 3,200 years, compared to 50-200years for CO₂) together with its high potency as a greenhouse gas(23,900 times the GWP₁₀₀ of CO₂ on a mole basis) and has resulted in acall for voluntary reductions in emissions. Because of this, the use ofSF₆ is being restricted and it is expected to be banned in the nearfuture. In addition, SF₆ is a relatively expensive gas.

[0019] Some of the best alternatives to SF₆ for blanketing gases wouldbe perfluorocarbons, such as CF₄, C₂F₆, and C₃F₈, but these materialsalso have high GWP's. Other alternatives would be chlorofluorocarbons(CFC's) or partially fluorinated hydrocarbons (HCFC's). However, the useof CFC's and HCFC's also is restricted; most of these materials arebanned as ozone depleters under the Montreal Protocol.

[0020] Another alternative to SF₆ for a blanketing gas is SO₂. When SO₂is used as a blanketing gas, the effective concentration over a melt istypically in the range of about 30% to 70% S02, with about 50% beingnormal. However, as discussed earlier, SO₂ poses environmental andhealth problems, is toxic, and can cause explosions. In addition, theuse of SO₂ in such relatively high concentrations can cause corrosionproblems on furnace walls.

[0021] Even when metals and alloys containing high levels of nonferrousmetals, such as alloy AZ61 (5.5-6.5% Al, 0.2-1.0% Zn, 0.1-0.4% Mn,(balance Mg), are exposed to high temperatures for purposes of solutionheat treating, annealing, or in preparation for rolling, forging, orother processing, it has been found advantageous to protect the metal orthe shape with an atmosphere that will inhibit undesirable surfaceoxidation or ignition, as is taught in U.S. Pat. No. 6,079, 477.

[0022] It also has been found desirable to protect such metals andalloys when they are in a highly divided form, such as powders or chips,and are being fed into metals processing systems prior to melting, as istaught in International Publication No. WO 00/00311.

[0023] It is desired to have a process for preventing oxidation ofmolten metals and alloys which overcomes the difficulties anddisadvantages of the prior art to provide better and more advantageousresults.

[0024] It is further desired to have an improved method of processingmetals and alloys at elevated temperatures using blanketing gases havinglower global warming potentials than the gases used in prior artmethods.

[0025] It also is desired to have an improved method of processingmetals and alloys at elevated temperatures using blanketing gases whichovercomes the difficulties and disadvantages of the prior art to providebetter and more advantageous results.

BRIEF SUMMARY OF THE INVENTION

[0026] A first embodiment of the present invention is an improvement ina method of processing a nonferrous metal and alloys of the metal usinga blanketing gas having a global warming potential. The improvementcomprises reducing the global warming potential of the blanketing gas byblanketing the nonferrous metal and alloys with a gaseous mixtureincluding at least one compound selected from the group consisting ofCOF₂, CF₃COF, (CF₃)₂CO, F₃COF, F₂C(OF)₂, SO₂F₂, NF₃, SO₂ClF, SOF₂, SOF₄,NOF, F₂and SF₄.

[0027] There are several variations of the first embodiment of theimprovement in the method. In one variation, the at least one compoundis provided at a first concentration of less than about 10% on a molebasis of the gaseous mixture. In addition, there may be several variantsof that variation. In one variant, the first concentration is less thanabout 6%. In another variant, the first concentration is less than about3%. In yet another variant, the first concentration is greater thanabout 0.1% and less than about 1%.

[0028] In another variation, the gaseous mixture further comprises atleast one member selected from the group consisting of N₂, Ar, CO₂, SO₂and air. In a variant of that variation, the at least one member is CO₂provided at a second concentration of about 30% to about 60% on a molebasis. In a variant of that variant, the at least one compound isprovided at the first concentration of less than about 3% on a molebasis and is selected from the group consisting of SO₂F₂ and COF₂.

[0029] In yet another variation, the gaseous mixture used in the methodalso includes an odorant. And in another variation, at least a portionof the gaseous mixture is recovered for reuse.

[0030] In still yet another variation, the nonferrous metal and alloyshave a temperature of at least about 0.5×T_(melt) (in degrees Kelvin).In addition, there are several variants of this variation. In onevariant, the temperature is at least about 0.7×T_(melt) (in degreesKelvin). In another variant, the temperature is a solidus temperature ofthe metal and alloys. In yet another variant, the temperature is greaterthan a solidus temperature of the metal and alloys but less than aliquidus temperature of the metal and alloys. In still yet anothervariant, the temperature is greater than a liquidus temperature of themetal and alloys but less than about 2.0×T_(boiling) (in degreesKelvin).

[0031] Another aspect of the present invention is a method as in thefirst embodiment of the improvement in the method, wherein at least oneoperation is performed on the nonferrous metal and alloys, the at leastone operation being selected from the group consisting of melting,holding, alloying, ladling, stirring, pouring, casting, transferring andannealing of the nonferrous metal and alloys.

[0032] The present invention also includes an improvement in a method ofprocessing a melt comprising at least one nonferrous metal using ablanketing gas having a global warming potential. The improvementcomprises reducing the global warming potential of the blanketing gas byblanketing said melt with a gaseous mixture including at least onecompound selected from the group consisting of COF₂, CF₃COF, (CF₃)₂CO,F₃COF, F₂C(OF)₂, SO₂F₂, NF₃, SO₂ClF, SOF₂, SOF₄, NOF, F₂ and SF₄.

[0033] The present invention also includes a process for preventingoxidation of a nonferrous metal and alloys of the metal. A firstembodiment of the process includes blanketing the nonferrous metal andalloys with an atmosphere containing an effective amount of at least onecompound selected from the group consisting of COF₂, CF₃COF, (CF₃)₂CO,F₃COF, F₂C(COF)₂, SO₂F₂, NF₃, SO₂ClF, SOF₂, SOF₄, NOF, F₂ and SF₄.

[0034] There are several variations of the first embodiment of theprocess. In one variation, the at least one compound is provided at afirst concentration of less than about 10% on a mole basis of theatmosphere. In addition, there may be several variants of thatvariation. In one variant, the first concentration is less than about6%. In another variant, the first concentration is less than about 3%.In yet another variant, the first concentration is greater than about0.1% and less than about 1%.

[0035] In another variation, the atmosphere further comprises at leastone member selected from the group consisting of N₂, Ar, CO₂, SO₂ andair. In a variant of that variation, the at least one member is CO₂provided at a second concentration of about 30% to about 60% on a molebasis. In a variant of that variant, the at least one compound isprovided at the first concentration of less than about 3% on a molebasis and is selected from the group consisting of SO₂F₂ and COF₂.

[0036] In yet another variation, the atmosphere used in the process alsoincludes an odorant. And in another variation, at least a portion of theatmosphere is recovered for reuse.

[0037] In still yet another variation, the nonferrous metal and alloyshave a temperature of at least about 0.5×T_(melt) (in degrees Kelvin).In addition, there are several variants of this variation. In onevariant, the temperature is at least about 0.7×T_(melt) (in degreesKelvin). In another variant, the temperature is a solidus temperature ofthe metal and alloys. In yet another variant, the temperature is greaterthan a solidus temperature of the metal and alloys but less than aliquidus temperature of the metal and alloys. In still yet anothervariant, the temperature is greater than a liquidus temperature of themetal and alloys but less than about 2.0×T_(boiling) (in degreesKelvin).

[0038] Another aspect of the present invention is a process as in thefirst embodiment of the process, wherein at least one operation isperformed on the nonferrous metal and alloys, the at least one operationbeing selected from the group consisting of melting, holding, alloying,ladling, stirring, pouring, casting, transferring and annealing of thenonferrous metals and alloys.

[0039] The present invention also includes a process for preventingoxidation of a melt including at least one nonferrous metal, the processcomprising blanketing the melt with an atmosphere containing aneffective amount of at least one compound selected from the groupconsisting of COF₂, CF₃COF, (CF₃)₂CO, F₃COF, F₂C(OF)₂, SO₂F₂, NF₃,SO₂ClF, SOF₂, SOF₄, NOF, F₂ and SF₄.

DETAILED DESCRIPTION OF THE INVENTION

[0040] The invention provides a process for preventing oxidation ofnonferrous metals or alloys thereof by blanketing the metals or alloyswith an atmosphere containing an effective amount of at least onecompound having a reduced GWP, preferably selected from the groupconsisting of COF₂, CF₃COF, (CF₃)₂CO, F₃COF, F₂C(OF)₂, SO₂F₂, SOF₂,SOF₄, NF₃, SO₂ClF, NOF, F₂ and SF₄. The invention also provides animproved method of processing nonferrous metals and alloys thereof usinga blanketing gas having a reduced GWP (relative to the prior art) byblanketing the nonferrous metals or alloys with a gaseous mixtureincluding at least one compound having a reduced GWP, preferablyselected from the group consisting of COF₂, CF₃COF, (CF₃)₂CO, F₃COF,F₂C(OF)₂, SO₂F₂, SOF₂, SOF₄, NF₃, SO₂ClF, NOF, F₂ and SF₄.

[0041] The invention may be applied in many types of operations,including but not limited to the melting, holding, alloying, ladling,stirring, pouring, casting, transferring and annealing of nonferrousmetals and alloys thereof. Additional applications include suchoperations as cladding, plating, rolling, protecting scrap whencompacting, preparing powder for improved alloying, protecting reactivemetals during electric arc spray coating or any other thermal spraycoating, fusing, brazing, and joining/welding operations, and improvingthe corrosion and wear resistance of articles of magnesium or magnesiumbased alloys. Persons skilled in the art will recognize other operationswhere the invention also may be applied.

[0042] The gases used in the present invention have lower GWP's than thegases used in the prior art and/or provide greater protection tooperators under operating conditions that utilize lower concentrationsof the gases. Since the gases used in the present invention are morereactive than SF₆, these gases can be used at concentrations supplyingan equivalent or lower fluorine level. In other words, if SF₆ can bebeneficially used at a concentration in the range of about 0.3% to about1%, then SO₂F₂ will have a similar utility at concentrations from about0.2% to about 3%.

[0043] In a preferred embodiment, the selected compound is provided at aconcentration of less than about 10% (on a mole basis) of said gaseousmixture. It is more preferable that the concentration be less than about6%, and it is even more preferable that it be less than about 3%.

[0044] However, since F₂, ClF, and ClF₃ are much more reactive than theother gases used in the present invention, these gases (F₂, ClF andClF₃) should only be used at lower concentrations, i.e., at aconcentration less than 5% and preferably less than 1%. In particular,if used at higher concentrations (e.g., 10%) in connection with a moltenor hot metal, these gases (F₂, ClF and ClF₃) may ignite and cause ametal/fluorine fire. Also, as shown in Table 1 below, F₂, ClF and ClF₃are very toxic. These gases will react relatively indiscriminately withany surfaces exposed to any of these gases, such as iron/steelstructures used in melt processes (e.g., melt pots, furnaces, etc.).This could result in relatively thick metal fluoride layers that mayincrease the risk of “thermite” type reactions, generation of HF uponexposure to atmospheric moisture, and HF burns to operators due toaccidental contact with metal fluoride layers.

[0045] In a preferred embodiment, the gaseous mixture further comprisesat least one member selected from the group consisting of N₂, Ar, CO₂and air as a diluent. SO₂ also could be used as the diluent, but is lessdesirable because of potential corrosion problems associated with SO₂.In addition, F₂ is violently reactive with SO₂, which would make itextremely dangerous to use SO₂ as a diluent if F₂ is present above tracelevels.

[0046] The most efficacious mixtures for blanketing nonferrous metalscontain significant concentrations of CO₂, preferably in the range ofabout 30% to about 60%. Some nonferrous metals also could benefit fromthe addition of chlorine or chlorine-containing species (such asSO₂—ClF) to the blanketing gas mixture.

[0047] For example, in one embodiment, CO₂ is the diluent in theblanketing atmosphere at a concentration of about 30% to about 60% on amole basis, and SO₂F₂ is provided at a concentration of less than about3% on a mole basis. In another embodiment, CO₂ is the diluent in theblanketing atmosphere at a concentration of about 30% to about 60% on amole basis, and COF₂, either alone or in combination with SO₂F₂, isprovided in a concentration of less than about 3% on a mole basis(referring to COF₂).

[0048] In a preferred embodiment, an odorant is added for safetypurposes to the mixture used for the blanketing atmosphere. This isespecially preferred for odorless gases, such as SO₂F₂. In contrast,since F₂, SOF₂ and SF₄ have distinctive odors, the addition of anodorant is less important when these gases are used. The same is truewhen SO₂ is used as a diluent because of the odor of SO₂.

[0049] Table 1 compares the preferred gases used in the presentinvention to various gases used in the prior art with regard to GWP andother characteristics. Several gases which technically could be used inthe present invention, but are likely to be too expensive or tooreactive to use, include ClF, ClF₃, CF₃COCl, (CF₃)₂NH, and CF₂(O)CFCF₃.TABLE I OSHA PEL/ Atmospheric Odor CAS Ceiling/ ACGIH Lifetime(detection Name Formula Number⁽¹⁾ Max Peak⁽²⁾ TWA/STEL⁽³⁾ GWIP₁₀₀ ⁽⁴⁾years limit in ppm) Sulfur SF₆ 2551-62-4 1,000/x/x 1,000/1,250 24,9003,200 Odorless Hexafluoride Sulfur Dioxide SO₂ 7446-09-5 215/x 10/15−1⁽⁵⁾ NK⁽⁶⁾ Irritating Acid (3-5) Carbon Dioxide CO₂ 124-38-95,000/30,000 asphyxiant 1 50-200 Odorless Perfluoromethane CF₄ 75-73-0 Xasphyxiant 6,500 50,000 Odorless Perfluoroethane C₂F₆ 76-16-4 Xasphyxiant 9,200 to 10,000 Odorless 12,500 Perfluoropropane C₃F₈ 76-19-7X asphyxiant 6,950 7,000 Odorless Sulfuryl Fluoride SO₂F₂ 2699-79-85/10/x toxic ˜1 NK Odorless Thionyl Fluoride SOF₂ 7783-84-8 X toxic ˜1NK Suffocating Sulfinyl Fluoride Sulfur Oxifluoride SO₂F₄ 13709-54-1 Xtoxic ˜1 NK NK Sulfur SF₄ 7783-60-0 x/0.1/x 0.1/0.3 ˜1 NK Like SO₂Tetraflouride Nitrogen NF₃ 7783-54-2 10/x/x 10/15 8,000 to 180 to 740Moldy Triflouride 9,720 Nitrosyl Fluoride NOF 7789-25-5 X toxic ˜1 NK NKSulfuryl Chloride SO₂CIF 13637-84-8 X toxic ˜1 NK NK Fluoride CarbonylCOF₂ 353-50-4 2/5 2/5 ˜1 50-200 Sharp HF Fluoride Irritating Trifluoroacetyl CF₃COF 354-34-7 X toxic NK NK NK Fluoride hydrolizes Trifuoroacetyl CF₃COCl 354-32-5 X toxic NK NK NK chloride hydrolizesHexafluoro-acetone (CF₃)₂CO 684-16-2 X toxic NK NK NA⁽⁷⁾ 0.1 PPM skinHexafluoro-acetone (CF₃)₂NH 1645-75-6 X toxic NK NK NA Fluoroxy- F₃COF373-91-1 X toxic ˜1 50-200 Sharp HF trifluoromethane hydrolizes toIrritating CO₂ Bisfluoroxy- F₂C(OF)₂ 16282-67-0 X toxic ˜1 50-200 SharpHF difluoromethane hydrolizes to Irritating CO₂ Hexafluoro-propeneCF₂(O)CFCF₃ 428-59-1 X toxic NK NK NA oxide Fluorine F₂ 7782-41-4 0.11/2 ˜0 <1 Sharp hydrolizes Pungent Irritating Chlorine Cl₂ 7782-50-50.5/1.0 1/3 ˜0 <1 Disagreeable hydrolizes Suffocating Chlorine FluorideCIF 7790-89-8 Not toxic ˜0 <1 Acid established hydrolizes Halogen toxicodor VERY sharp pungent Chlorine CIF₃ 7790-91-2 /0.1 /0.1 ˜0 <1 SweetTrifluoride hydrolizes Suffocating

[0050] The comparison of GWP₁₀₀ shows that ten of the thirteen preferredgases used in the present invention (COF₂, CF3COF, (CF3)₂CO, F₃COF,F₂C(OF)₂, SO₂F₂, NF₃, SO₂ClF, SF₄, SOF₂ NOF, F₂ and SOF₄) havesignificantly lower GWP₁₀₀'s than the gases used in the prior art. (Ofthe thirteen gases, only NF₃ has a GWP₁₀₀ greater than ˜1; but theGWP₁₀₀ of NF₃ is still several fold lower than the GWP₁₀₀ of SF₆, andthe atmospheric life of NF₃ also is shorter than that of SF₆. For two ofthe other gases, CF₃ COF and (CF₃)₂CO, the GWP₁₀₀'s are not known.)Furthermore, the prior art did not teach or even appreciate the possibleuse of these gases for blanketing. For example, the IMA TechnicalCommittee Report shows that SO₂F₂ and SF₄ are by-products of the SF₆protective chemistry for magnesium, but that report fails to realizethat both SO₂F₂ and SF₄ can be potent sources of fluorine for protectionof the melt. The gases used in the present invention may be recoveredand recycled for reuse. Recovery techniques that may be used include theuse of membranes, absorption, condensing and other means to concentratethe desirable gases for reuse.

[0051] While the present invention has been described in detail withreference to certain specific embodiments, the invention is neverthelessnot intended to be limited to the details described. Rather, it will beapparent to persons skilled in the art that various changes andmodifications can be made in the details within the scope and range ofthe claims and without departing from the spirit of the invention andthe scope of the claims.

1. In a method of processing a nonferrous metal and alloys of said metalusing a blanketing gas having a global warming potential, theimprovement comprising reducing said global warming potential of saidblanketing gas by blanketing said nonferrous metal and alloys with agaseous mixture including at least one compound selected from the groupconsisting of COF₂, CF₃COF, (CF₃)₂CO, F₃COF, F₂C(OF)₂, SO₂F₂, NF₃,SO₂ClF, SOF₂, SOF₄, NOF, F₂ and SF₄.
 2. A method as in claim 1 , whereinsaid at least one compound is provided at a first concentration of lessthan about 10% on a mole basis of said gaseous mixture.
 3. A method asin claim 2 , wherein said first concentration is less than about 6%. 4.A method as in claim 2 , wherein said first concentration is less thanabout 3%.
 5. A method as in claim 2 , wherein said first concentrationis greater than about 0.1% and less than about 1%.
 6. A method as inclaim 2 , wherein said gaseous mixture further comprises at least onemember selected from the group consisting of N₂, Ar, CO₂, SO₂ and air.7. A method as in claim 6 , wherein said at least one member is CO₂provided at a second concentration of about 30% to about 60% on a molebasis.
 8. A method as in claim 7 , wherein said at least one compound isprovided at said first concentration of less than about 3% on a molebasis and is selected from the group consisting of SO₂F₂ and COF₂
 9. Amethod as in claim 1 , wherein at least one operation is performed onsaid nonferrous metal and alloys, said at least one operation beingselected from the group consisting of melting, holding, alloying,ladling, stirring, pouring, casting, transferring and annealing of saidnonferrous metal and alloys.
 10. A method as in claim 1 , wherein saidnonferrous metal and alloys have a temperature of at least about0.5×T_(melt) (in degrees Kelvin).
 11. A method as in claim 10 , whereinsaid temperature is at least about 0.7×T_(melt) (in degrees Kelvin). 12.A method as in claim 10 , wherein said temperature is a solidustemperature of said metal and alloys.
 13. A method as in claim 10 ,wherein said temperature is greater than a solidus temperature of saidmetal and alloys but less than a liquidus temperature of said metal andalloys.
 14. A method as in claim 10 , wherein said temperature isgreater than a liquidus temperature of said metal and alloys but lessthan about 2.0×T_(boiling) (in degrees Kelvin).
 15. A method as in claim1 , wherein said gaseous mixture further comprises an odorant.
 16. Amethod as in claim 1 , wherein at least a portion of said gaseousmixture is recovered for reuse.
 17. In a method of processing a meltcomprising at least one nonferrous metal using a blanketing gas having aglobal warming potential, the improvement comprising reducing saidglobal warming potential of said blanketing gas by blanketing said meltwith a gaseous mixture including at least one compound selected from thegroup consisting of COF₂, CF₃COF, (CF₃)₂CO, F₃COF, F₂C(OF)₂, SO₂F₂, NF₃,SO₂ClF, SOF₂, SOF₄, NOF, F₂ and SF₄.
 18. A process for preventingoxidation of a nonferrous metal and alloys of said metal comprisingblanketing said nonferrous metal and alloys with an atmospherecontaining an effective amount of at least one compound selected fromthe group consisting of COF₂, CF₃COF, (CF₃)₂CO, F₃COF, F₂C(OF)₂, SO₂F₂,NF₃, SO₂ClF, SOF₂, SOF₄, NOF, F₂ and SF₄.
 19. A process as in claim 18 ,wherein said at least one compound is provided at a first concentrationof less than about 10% on a mole basis of said atmosphere.
 20. A processas in claim 19 , wherein said first concentration is less than about 6%.21. A process as in claim 19 , wherein said first concentration is lessthan about 3%.
 22. A process as in claim 19 , wherein said firstconcentration is greater than about 0.1% and less than about 1%.
 23. Aprocess as in claim 19 , wherein said atmosphere further comprises atleast one member selected from the group consisting of N₂, Ar, CO₂, SO₂and air.
 24. A process as in claim 23 , wherein said at least one memberis CO₂ provided at a second concentration of about 30% to about 60% on amole basis.
 25. A process as in claim 24 , wherein said at least onecompound is provided at said first concentration of less than about 3%on a mole basis and is selected from the group consisting of SO₂F₂ andCOF₂.
 26. A process as in claim 18 , wherein at least one operation isperformed on said nonferrous metal and alloys, said at least oneoperation being selected from the group consisting of melting, holding,alloying, ladling, stirring, pouring, casting, transferring andannealing of said nonferrous metal and alloys.
 27. A process as in claim18 , wherein said nonferrous metal and alloys have a temperature of atleast about 0.5×T_(melt) (in degrees Kelvin).
 28. A process as in claim27 , wherein said temperature is at least about 0.7×T_(melt) (in degreesKelvin).
 29. A process as in claim 27 , wherein said temperature is asolidus temperature of said metal and alloys.
 30. A process as in claim27 , wherein said temperature is greater than a solidus temperature ofsaid metal and alloys but less than a liquidus temperature of said metaland alloys.
 31. A process as in claim 27 , wherein said temperature isgreater than a liquidus temperature of said metal and alloys but lessthan about 2.0×T_(boiling) (in degrees Kelvin).
 32. A process as inclaim 18 , wherein said atmosphere further comprises an odorant.
 33. Aprocess as in claim 18 , wherein at least a portion of said atmosphereis recovered for reuse.
 34. A process for preventing oxidation of a meltcomprising at least one nonferrous metal, said process comprisingblanketing said melt with an atmosphere containing an effective amountof at least one compound selected from the group consisting of COF₂,CF₃COF, (CF₃)₂CO, F₃COF, F₂C(OF)₂, SO₂F₂, NF₃, SO₂ClF, SOF₂, SOF₄, NOF,F₂ and SF₄.